1. Field of the Invention
Apparatus and method for forming tubular tapered shafts.
2. Description of the Prior Art
In the past, attempts have been made to mold a tubular tapered shaft by pressure in an annulus shaped space defined between the interior surface of an elongate tubular shell and the exterior surface of an elongate tapered core. In this process as disclosed in the Robert F. Pigg U.S. Pat. No. 2,768,921 entitled Method of Molding Tapered Rods that issued Oct. 30, 1956 the resulting tapered tubular shaft is of poor quality as no means are provided to maintain the core and shell in true concentric relationship with one another. Furthermore, in the Pigg method the entire annulus shaped space is filled with a resin impregnated glass cloth, and this cloth remaining stationary relative to the shell as the resin polymerizes. The core in the Pigg method moves longitudinally relative to the resin impregnated material in the annulus space, and as a result the interior portion of the Pigg tubular shaft is strong and the exterior portion weak.
This strength of the inner portion of the Pigg tubular shaft is due to at least a portion of the fibers of the glass cloth being pre-tensioned prior to polymerization of the resin due to movement of the core relative to the shell. The outer portion of the Pigg tubular shaft is weak due to the resin impregnated material remaining stationary relative to the shell and there accordingly being no pre-tensioning of the glass fibers therein. Pre-tensioning of the maximum amount of fibers in the polymerized resin is necessary if the finished tubular shaft is to have substantial strength, for it is these pre-tensioned fibers that absorb the major portion of the load when the tubular shaft is flexed transversely as occurs when the shaft is used in a golf club or fishing rod.
If the resin enveloping untensioned fibers is more elastic than the fibers, the resin at the initial flexing of the tubular shaft will take all of the load, and may deform as a result thereof to the extent that it will permanently separate from the fibers. Subsequent flexing of the tubular shaft will cause relative movement between the resin and fibers, and the fibers being of a harder material will cut through the resin to contact adjacently disposed fibers. The breaking of the bonds between the fibers and resin to permit the above described cutting action as the shaft flexes results in rapid deterioration of the shaft which is evidenced by cracks and other physical imperfections appearing therein.
The primary purpose in devising the present invention is to provide an apparatus and method of using the same to form a tubular tapered shaft that is defined by truly concentric exterior and interior sidewall surfaces, and the material from which the shaft is formed being a thermosetting resin that has a number of spaced pre-tensioned fibers developed therein that absorb a major portion of the load as soon as the shaft is initially flexed in a transverse direction.
A further object of the invention is to provide a method by which a tapered tubular shaft is so molded from a thermosetting resin that contracts upon polymerization that the resin is concurrently cured from both the exterior and interior thereof, with the tubular shaft during the application of heat moving longitudinally relative to a tapered mold due to spring loading on a tapered core, and the spring loading being so controlled that spaced elongate graphite carbon or boron fibers enveloped in the resin are pretensioned but not to the extent that the fibers have the resin separating them displaced therefrom to permit the fibers to physically contact one another.